Clouds disappear during a solar eclipse, and we finally know why: ScienceAlert

When the moon passes in front of the sun in a solar eclipse, this stunning spectacle appears to momentarily change our world.

However, the effects on our planet go much deeper than a few moments of darkness during daylight hours. And one effect that might surprise you? The clouds dissipate quickly – from the point where the Moon blocks only 15 percent of the Sun.

Obviously it's not all types of clouds; Otherwise we would never hear complaints about eclipses Spoiled by overcast weather.

A team led by Victor Trees of the Royal Netherlands Meteorological Institute and Delft University of Technology has found that shallow cumulonimbus clouds, in particular, float above the ground quickly.

This discovery has implications for future attempts at climate engineering, Trees says.

“If we eclipse the sun in the future with technological solutions, this may affect the clouds.” He explains. “Reduced cloud numbers may partially counteract the intended effect of climate engineering, because clouds reflect sunlight and thus actually help cool the Earth.”

Evolution of cloud cover during the 2005 solar eclipse. The right column shows cloud cover after correcting for the Moon's shadow. (trees and others, subscriber. Earth environment., 2005)

From our perspective here on Earth, knowing how clouds behave during an eclipse is not easy.

But Trees says it's important: One proposed solution to mitigate climate change is to block some sunlight from reaching Earth's lower atmosphere. While modeling suggests this could effectively lower temperatures, we don't know what other effects it might have.

Because cloud layers can be so complex, and because cloud formations are constantly moving, calculating clouds from the ground is not actually a viable way to determine how sun dimming affects clouds.

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Another option is to study it from above using satellites, but previously those satellites did not take the moon's shadow during an eclipse into account in cloud top reflectance calculations, leading to bias in measurements of cloud cover and thickness.

Trees and his colleagues discovered a way to correct for the Moon's shadow by taking into account what proportion of the Sun is obscured at any given time, from each location on Earth's surface.

“By far, most solar eclipses consist of partial eclipses, where there is still plenty of light outside.” The trees say. “In this partial eclipse, the satellites receive enough reflected sunlight, after correcting for occultation, to reliably measure clouds.”

Results of the team's model of cloud evolution during a solar eclipse. (trees and others, subscriber. Earth environment., 2024)

The researchers applied their methods to data collected during three previous solar eclipses over the African continent, between 2005 and 2016. To their surprise, the cumulonimbus clouds began to disappear in large numbers when only 15% of the Sun was covered, disappearing until the eclipse. I finish.

It wasn't clear why this happened, so the team ran simulations using a cloud modeling program called Deals. These simulations showed that when sunlight is blocked, the surface cools, reducing updrafts of warm air from the surface.

Warm updrafts are Useful in forming cumulus clouds; They carry water vapor that condenses into droplets when they rise to cooler altitudes, forming clouds.

Therefore, when the Earth cools, and these updrafts stop, the cumulonimbus clouds cannot continue, and they start again only when the sun reappears and begins to heat the Earth again. This effect only occurs on land, because the ocean does not cool quickly enough for the effect to start.

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Cumulus clouds are Not the rain clouds themselvesBut it could turn into rain clouds. The team's findings suggest that climate geoengineering that involves blocking sunlight could have a very harmful effect on weather patterns.

Since this is the opposite of what scientists want to happen, this phenomenon requires more research, the researchers say.

The research was published in Earth and Environment Communications.

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